A CLOSED LOOP CONTROL STRATEGY FOR A FREQUENCY-ONLY CONVERTER WITH HIGH EFFICIENCY

Many electric power generators use gas turbines as power sources, coupled through a mechanical gearbox in order to adapt their synchronous speed to the optimal rotation speed of the turbine. This paper proposes to replace the mechanical gearbox with a flexible electronic solution which would offer a higher efficiency. A new control strategy is proposed for the three level Neutral Point Clamped converter, which is characterized by its high efficiency due to the use of square-wave modulation. The main advantage of this modulation is the quasi absence of switching losses. In this modulation, only the frequency can be adapted between the input and the output voltages, but their magnitudes are not freely controllable. The active and reactive powers produced can be controlled by the generator’s excitation, as well as both the angle shift between the generator’s and rectifiers's voltages and between the inverter’s and network’s voltages. The simulation and a partial experimental results for different operating points highlight the capabilities of the proposed control strategy.

Gas Turbine: Optimization of Energy Production and High Efficiency by Using Power Electronics

Aziza Benaboud, Alfred Rufer

The use of gas turbines in the power generation industry has been increasing. Their demand represents more than 50% of the world market of thermal power plants. Electric power generators using gas turbines as power sources are connected to the turbines through a mechanical gearbox, in order to adapt their synchronous speed to the optimal rotation speed of the turbine, which is very often much higher than the synchronous speed. However, due to direct network connection, the generator speed cannot be variable: it is imposed by the network and constant. To overcome this problem, we propose to replace the mechanical gearbox by a flexible electronic solution which offers a high efficiency. Using this approach, the turbine is directly connected to the synchronous generator, which is connected to the grid through an indirect static frequency converter. However, this type of converter is not common in this application because of very high switching losses due to the high frequency of the PWM technique used for its control. In this paper, a new control strategy is proposed for the multi-level converter, characterized by its high efficiency due to the use of square-wave modulation. The main advantage of this mode is the quasi absence of switching losses. In this mode, only the frequency can be varied between the input and the output voltage, but their magnitudes are not freely controllable. A voltage magnitude adaptation can be done by the generator's excitation. The produced active and reactive power can be controlled by the generator excitation as well as both the angle shift between the generator and rectifier voltages and between the inverter and network voltages. Simulation and experimental results for different operating points highlight the capabilities of the proposed control strategy. These include the ability to operate with unity power factor and better current quality. (C) 2016 The Authors. Published by Elsevier Ltd.

Elsevier Science Bv2016

Convertisseur de fréquence indirect à rapport de tension fixe

Aziza Benaboud

Many electric power generators use gas turbines as power sources. Typically they are connected to the turbines through a mechanical gearbox in order to adapt their synchronous speed to the optimal rotation speed of the turbine, which is very often much higher than the synchronous speed. However, due to direct network connection, the generator speed cannot be variable : it is imposed by the network and constant. To overcome this problem, we propose to replace the mechanical gearbox by a flexible electronic solution which offers a high efficiency. Using this approach, the turbine is directly connected to the synchronous generator, which is connected to the grid through an indirect static frequency converter with an intermediary DC circuit. However, this type of converter is not common in this application because of very high switching losses due to the high frequency of the PWM technique normally used for its control. In this dissertation, a new control strategy is proposed for the three level Neutral Point Clamped converter, characterized by its high efficiency due to the use of square-wave operation mode. The main advantage of this mode is the quasi absence of switching losses. In this mode, only the frequency can be varied between the input and the output voltage, but their magnitudes are not freely controllable. A voltage magnitude adaptation can be done by the generator's excitation. The produced active and reactive power can be controlled by the generator excitation as well as both the angle shift between the generator and rectifier voltages and between the inverter and network voltages. The capacitive intermediary circuit brings the advantage of decoupling of harmonics between the generator and the network currents. A control method is also proposed to resolve some problems incurred by using square wave operation mode, in particular to reduce the harmonics distortion of the output inverter voltage and current. As second important contribution, this thesis proposes a new fast-ramping DC-component elimination strategy for AC currents. In comparison to the usually slow transient that characterizes a DC component free current transient, we achieve that much faster transients also without DC-component, simply by choosing a well defined transition period. Simulation and experimental results for different operating points and transitions between them highlight the capabilities of the proposed control strategy. These include the ability to operate with unity power factor and better current quality without continuous component and less harmonics.

EPFL2007

New Control Strategy for Three Level NPC Converter Connected to the Grid

Aziza Benaboud, Alfred Rufer

The aim of the present paper is to describe a new control strategy for a three level NPC converter connected to the grid, characterized by its high efficiency. This efficiency is achieved using a square wave modulation or SWM, which has the advantage of a quasi absence of switching losses. The produced active and reactive power can be controlled by voltage magnitude adaptation in the input of the inverter, together with a phase shift between the grid and output inverter voltages. Therefore, the phase-locked loop (PLL) can be considered to be an important part of the power generation system connected to the grid. The contributions of the paper include an approach to achieve a robust PLL, a discussion of the properties and operation performance of the new control strategy, as well as the corresponding characteristic curves. The performance will be illustrated through simulation examples as well as experimental results.

2006

Gas Turbine: Optimization of Energy Production and High Efficiency by Using Power Electronics

Aziza Benaboud, Alfred Rufer

Elsevier Science Bv2016

Convertisseur de fréquence indirect à rapport de tension fixe

Aziza Benaboud

EPFL2007